1. Field of the Invention
The present invention relates to a communication system and, more particularly, to a high speed data communication and method.
2. Background of the Related Art
Recently, mobile communication systems provide high quality Internet service, mobile image contents service such as VOD, and a personal-centered service including a credit card, diverse identifications and electronic seal impression function, as well as a voice service provided in the next-generation mobile communication system. Accordingly, in order to provide multimedia service to meet various demands of users, attention is increasingly paid to the mobile communication system and its technological development.
CDMA 2000, currently available for high speed data transmission, uses a spread spectrum communication technique that can be roughly divided into a CDMA 2000 1× and 1×EV-DO. CDMA 2000 1× is a service supportable up to 153.6 kbps, which is far faster than 9.6 kbps or 65 kbps, the rate supported in IS-95A or IS-95B networks. CDMA 20001× uses the IS-95C network developed from IS-95A and IS-95B. It can support diverse multimedia services as well as improve voice and Wireless Application Protocol (WAP) service quality. In addition, its coverage is extended to business, electronic commercial transactions, and a high-tech industrial fields beyond the personal communications.
1×EV-DO (1× Evolution-Data Optimized), a packet data-only protocol optimized for a high speed/high capacity data transmission, transmits data to a user by using a CDMA channel, having the same frequency band as that of the CDMA 2000×1. 1×EV-DO accomplishes a data rate of about 2.4 Mbps, which is 16 times faster when compared to 153.6 kbps of a forward channel of CDMA 2000 1× and has about 5 times greater capacity than CDMA 2000 1×. Thus, 1×EV-DO can provide a high speed data service simultaneously to numerous users. With the enhanced data transfer rate and capacity, 1×EV-DO provides diverse multimedia content such as radio Internet access, real-time traffic information, Internet game, M-commerce, and the like. 1×EV-DO maximizes a use efficiency of a radio section and a system by dynamically allocating a transfer rate of each user in the radio section and utilizing at the maximum a time slot of a packet data.
FIG. 1 illustrates a structure of a receiving apparatus of a base station of a mobile communication system in accordance with a conventional art, which can be also applied to the CDMA-2000 1× and 1×EV-DO system. The conventional receiving apparatus of the base station of a mobile communication system includes: a low noise amplifier (LNA) 1 for amplifying an RF signal received through an antenna from outside; a first band pass filter 2 for removing a spurious wave component of a signal outputted from the LNA 1; a frequency mixer 3 for mixing an output signal of the first band pass filter 2 with an output signal of a voltage control oscillator 9 and outputting an IF signal; a second band pass filter 4 for filtering a specific part of the output signal of the frequency mixer 3; a demodulator 5 for outputting an output signal of the second band pass filter 5 as separate signal ‘I’ and signal ‘Q’; and an MSM (mobile station modem) 7 for converting the I/Q signal of the demodulator 5 into a digital signal and reproducing an original signal.
The frequency mixer 3 mixes an output signal of the voltage control oscillator 9 under the control of a phase locked loop 8 and the RF signal outputted from the first band pass filter 2 and outputs an IF signal. The MSM 7 includes an analog/digital converter 10 for converting the IQ signal to a digital signal when the I/Q signal outputted from the demodulator 5 is inputted thereto after being filtered through a low pass filter 6. The digital signal outputted from the analog/digital converter 10 is reproduced to its original signal in the MSM 7.
The operation of the receiving apparatus of the base station in the mobile communication system will now be described. The RF signal inputted through the antenna is amplified by the low noise amplifier 1, and as the amplified RF signal passes the first band pass filter 2, its spurious wave component is removed. The spurious wave component-removed signal is inputted to the frequency mixer 3, where it is mixed with a signal inputted through the voltage control oscillator 9, to be outputted as the IF signal and then inputted to the second band pass filter 4.
The second band pass filter 4 filters the IF signal outputted from the frequency mixer 3 and inputs it to the demodulator 5. The IF signal inputted to the demodulator 5 is converted into the I/Q signal, inputted to the low pass filter 6 and filtered therein. Each I/Q signal outputted through the low pass filter 6 is converted into digital signals by the analog/digital converter 10 and is reproduced to an original signal through the MSM 7.
In general, in the case of the currently used 1×EV-DO system, the forward channel transmitting data from the base station to the mobile communication terminal communicates with 12 data rates, while a backward channel transmitting data from the mobile communication terminal to the base station communicates with 5 data rates. A backward data transfer rate is automatically controlled in and transmitted from the base station depending on a communication quality between the mobile communication terminal and the base station and the data transfer rate.
Every forward channel inputted as the I/Q data is time-divided and transmitted as a signal to a radio section. Various modulation methods are used for the forward channel according to a channel condition: a quadrature phase shift keying (QPSK) is used for 8 types of transfer rates from 38.4 kbps to 1228.8 kbps; from 924 kbps to 1843.2 kbps, 8 PSK (phase shift keying) is used; and at 1228.8 kbps and 2457.6 kbps 16 QAM (quadrature amplitude modulation) is used. That is, the modulation method is changed according to the data transfer rate, and the QPSK method is used for low speed data transfer, while the 16 QAM method is used for high speed transfer data.
FIGS. 2A, 2B and 2C illustrate general modulation methods used for 1×EV-DO. FIG. 2A shows the QPSK method. FIG. 2B shows the 8 PSK method. FIG. 2C shows the 16 QAM method. The QPSK method has 4 types of 2-bit digital signals 00, 01, 10 and 11 that are discriminately transmitted. Because each signal is apart from the origin with a predetermined distance with respect to the I/Q axis, a content of the signals can be judged only by the phase.
The 8 PSK method has 8 types of 3-bit digital signals 000, 001, 010, 011, 100, 101 and 110 are discriminately transmitted. As shown in FIG. 2B, a phase of a signal is modulated, for transmission, at 8 code pointers arranged at regular intervals on a circumference representing a phase of a carrier.
In the 16 QAM method, digital signals are classified as a certain quantity and modulated by changing a carrier signal and a phase. At this time, since the size as well as the phase is used as a variable, a large quantity of digital data can be simultaneously transmitted. However, this method is susceptible to error while decoding the transmittal signal.
FIG. 3 is a graph of a general signal determining theory showing a probability (P(Z/S1)) that a signal S1 is distributed at one point on the time axis and a probability (P(Z/S2)) that a signal S2 is distributed at one point on the time axis. As shown in FIG. 3, there is some error probability at a certain section on the basis of the time axis ‘0’. Little error is generated at the point P1 because there is no probability that S2 exists there. However, an error is generated at the point P2 because the actual signal S1 is recognized as the signal S2.
FIGS. 4A and 4B illustrate the same noise generated when the signal determining theory is adopted to the system of the QPSK method and the 16 QAM method. With reference to FIG. 4A, in the CDMA 2000 1× system using the QPSK method, data is transited as a phase error as much as ‘t’ is generated due to noise, but the data is judged not to have an error. With reference to FIG. 4B, in the ax EX-DO system using the 16 QAM method, if data is transited as a phase error is generated as much as ‘t’ due to noise, a probability is high in judging that there is an error in the data.
That is, assuming that the phase error generated due to various noises existing in the same receiving apparatus is the same, the high speed data communication using the 16 QAM is more sensitive to the error compared to the low speed data communication using the QPSK. Therefore, in the conventional receiving apparatus of high speed digital data communication systems, the phase error generation probability due to noise generation is high compared to the low speed data communication, not only the receive sensitivity is deteriorated but also the performance of the receiving apparatus is degraded.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.